1. Field of the Invention
This invention relates to a display device, especially to an electroluminescent display device with an electroluminescent element and a thin film transistor provided for each of display pixels.
2. Description of the Related Art
Display devices with an electroluminescent (referred to as EL hereinafter) element have been gathering attention as a display device substituting a CRT or an LCD. The development efforts for the EL display device with a thin film transistor (referred to as TFT hereinafter) as a switching element for driving the EL element have been made accordingly.
FIG. 3 is an equivalent circuit diagram of a display pixel of an EL display device with an EL element and a TFT. A plurality of the display pixels is disposed in a matrix configuration in an actual EL display device.
FIG. 3 is an equivalent circuit diagram of the EL display device having a first TFT 100, a second TFT 110 and the organic EL element 120, and shows only one display pixel of the matrix, which is located near the crossing of a gate signal line 50 at the n-th row and a drain signal line 60 at the m-th column.
The gate signal line, which supplies a gate signal Gn, and the drain signal line 60, which supplies a drain signal that is a video signal Dm, intersect each other. The organic EL element 120, the TFT 110 for driving the organic EL element, and the TFT 110 for selecting the display pixel are disposed near the crossing of these two signal lines.
A driving source 105, from which a positive driving voltage PVdd is supplied, is connected to a drain 100d of the first TFT 100 for driving the organic EL element. A source 100s is connected to an anode 121 of the organic EL element.
A gate 110g of the second TFT 110 for selecting the display pixel is connected to the gate signal line 50, receiving the gate signal Gn, and a drain 110d of the second TFT 110 is connected to the drain signal line 60, receiving the video signal Dm. A source 110s of the second TFT110 is connected to a gate 100g of the first TFT100. The gate signal Gn is outputted from a gate driver circuit not shown in the figure. The video signal Dm is outputted from a drain driver circuit not shown in the figure.
The organic EL element 120 includes the anode 121, a cathode 122, and a emission layer 123 disposed between the anode 121 and the cathode 122. The cathode 122 is connected to a common source 140 that supplies a negative common voltage.
A storage capacitance element 130 is connected to the gate 100g of the first TFT 100. That is, one of the electrodes of the storage capacitance element 130 is connected to the gate 100g, and the other electrode is connected to a storage capacitance electrode 131. The storage capacitance element 130 is provided in order to hold the video signal of the display pixel for one field period by keeping the charge corresponding to the video signal Dm.
The operation of the EL display device with the above configuration is as follows. The second TFT 100 turns on when the gate signal Gn becomes high-level for one horizontal period. Then, the video signal Dm is applied to the gate 100g of the first TFT 100 from the drain signal line 60 through the second TFT 110. The conductance of the first TFT 110 changes in response to the video signal Dm supplied to the gate 100g, and a driving current corresponding to the changed conductance is supplied to the organic EL element 120 from the driving source 105 through the first TFT 100. This controls the brightness of the organic EL element 120.
As described above, the brightness of the organic EL element 120 is controlled based on the conductance of the first TFT 100, which changes in response to the video signal Dm. However, conventional EL display devices lack an appropriate contrast in its display presentation.